As well known in the art, bonded magnets have many advantages such as light weight, good dimensional accuracy, and facilitated mass-production of molded products having even a complicated shape as compared to sintered magnets, and, therefore, have been extensively used in various applications such as toys, office equipments, audio equipments and motors.
As the magnetic particles usable in the bonded magnets, there are known rare earth element magnet particles such as typically Nd—Fe—B-based alloy particles, or ferrite particles. The rare earth element magnet particles have high magnetic properties, but are expensive, resulting in limited applications thereof. On the other hand, the ferrite particles are somewhat deteriorated in magnetic properties as compared to the rare earth element magnet particles, but are inexpensive and chemically stable and, therefore, have been used in more extensive applications.
The bonded magnets have been usually produced by kneading a rubber or plastic material with magnetic particles and then molding the resultant kneaded material in a magnetic field or by using a mechanical means.
In recent years, with the increase in performance of various materials or equipments including an improved productivity and an enhanced reliability upon use, there is also an increasing demand for a high performance of bonded magnets used therein including improvement in productivity as well as enhancement in mechanical strength and magnetic properties of the bonded magnets.
More specifically, upon production of the bonded magnets by injection-molding, etc., in order to not only improve general characteristics but also enhance a production efficiency thereof by increasing an injection frequency per unit time, in other words, in order to enhance the productivity, it is required to rapidly cooling the resulting molded product immediately after the injection-molding. Therefore, it is necessary to use a material having an excellent quenching resistance for production of bonded magnets such that the rapid cooling has no adverse influence on strength of the obtained bonded magnets. Further, the resulting bonded magnets are also required to exhibit a good mechanical strength capable of withstanding severe conditions when used in various applications.
For example, in copying machines, printers or the like using a magnet roll, since the magnet roll is used at a high speed in these equipments with a long service life, it has been strongly required that the magnet roll exhibits a high mechanical strength. In addition, as to the magnetic properties of the magnet roll, it has been especially required to achieve not only increase in a surface magnetic force of the magnet roll, but also uniformity in the surface magnetic force as important technical factors for obtaining clear images in the copying machines, printers, etc.
For this reason, ferrite particles used in the bonded magnets as well as resin compositions for the bonded magnets which comprise the ferrite particles and an organic binder are also required to satisfy the above requirements.
Conventionally, ferrite particles for bonded magnets and resin compositions for bonded magnets comprising the ferrite particles and the organic binder have been improved variously. For example, there are known the method of producing ferrite particles by using an alkali metal compound or an alkali earth metal compound as a flux (Patent Document 1); the method of treating ferrite particles with an acid and then treating the surface of the thus acid-treated ferrite particles with a phosphate-based coupling agent (Patent Document 2); the method of treating the surface of ferrite particles with a phosphoric acid compound (Patent Document 3); the method of treating ferrite particles with an alkali and then treating the thus alkali-treated ferrite particles with a coupling agent (Patent Document 4); the method of controlling a particle size distribution of ferrite particles (Patent Document 5); the method of producing a bonded magnet using ferrite magnetic particles comprising an alkali earth metal as a constituting component and having an average particle diameter of not less than 1.50 μm and a melt flow rate of not less than 91 g/10 min (Patent Document 6); or the like.
Patent Document 1: Japanese Patent Application Laid-Open (KOKAI) No. 55-145303
Patent Document 2: Japanese Patent Application Laid-Open (KOKAI) No. 4-93002
Patent Document 3: Japanese Patent Application Laid-Open (KOKAI) No. 2007-281381
Patent Document 4: Japanese Patent Application Laid-Open (KOKAI) No. 5-41314
Patent Document 5: Japanese Patent Application Laid-Open (KOKAI) No. 3-218606
Patent Document 6: Japanese Patent Application Laid-Open (KOKAI) No. 2005-268729